1. Field of the Invention
The present invention relates to projection exposure apparatus and exposure methods, for example, for fabricating semiconductor devices, liquid-crystal display devices, etc. by photolithography and, more particularly, to projection exposure apparatus provided with a projection optical system comprised of catadioptric optics or catoptric optics, and exposure methods using such apparatus.
2. Related Background Art
In the photography step for fabrication of the semiconductor devices and other devices, the projection exposure systems are used for printing a pattern image of a photomask or a reticle (which will be referred to hereinafter together as xe2x80x9creticlexe2x80x9d) through a projection optical system on a wafer (or a glass plate or the like) coated with a photoresist or the like.
The resolving power required of the projection optical system used in such projection exposure systems has been becoming higher and higher with increase in integration of the semiconductor devices and other devices. For meeting this demand, there arise the needs for decrease in the wavelength of illumination light and for increase in the numerical aperture (N.A.) of the projection optical system.
However, the decrease in the wavelength of illumination light leads to limitation of kinds of glass materials applicable to practical use because of absorption of light. The glass materials practically applicable at present in the wavelength range of not more than 300 nm are only synthetic silica and fluorite, and for this reason, there are desires for use of catoptric optics in order to effect correction for chromatic aberration. Specifically, practical use has considerably been developed using dioptric optics with the KrF laser of 248 nm, but it is very difficult to realize practical use using dioptric optics in the wavelength range of not more than 200 nm. Therefore, expectations are rising for the catadioptric optics. Particularly, it is known that fluorite has sufficient transmittance even at 100 nm, and since it can be used as a refracting member above this range, the catadioptric optics can be constructed in the range of wavelengths 100 to 300 nm.
Several types of the catadioptric optics have been proposed heretofore. Among them, an optical system of a type wherein the central part of N.A. is shielded (which will be referred to hereinafter as a center shield type) is a promising type, because all optical elements can be assembled on the basis of one optical axis without inclusion of a path deflecting member by use of two or more reflective surfaces and because it has a merit of capability of imaging an object on the optical axis on the image plane and thus correcting for aberration in a wide exposure field by a small number of optical elements. Prior arts of this type include those disclosed, for example, in U.S. Pat. No. 5,717,518, No. 5,650,877, and so on.
In general, the illumination light radiates to the projection optical system during exposure in the projection exposure apparatus, so as to bring about absorption of the illumination light (or exposure light), and the optical members undergo, for example, asymmetric deformation, internal temperature distribution, etc., thus causing variation in aberration (see Japanese Patent Application Laid-Open No. H09-213611).
This aberration variation is considered to be not only due to absorption inside the glass materials of the projection optical system, but also due to absorption in thin films or the like on surfaces. Particularly, in the catadioptric optics, absorption at the reflective surfaces is considered to be especially larger than that inside the glass materials and in the surface thin films, and the aberration variation due to absorption can be more problematic than in the case of the dioptric optics.
An effective means for the projection optical systems in the extreme ultraviolet region is use of a back reflector that causes reflection on a back surface of a refracting member, for example, as described in aforementioned U.S. Pat. No. 5,717,518. In this case, materials used need to be glass materials capable of transmitting light in this wavelength range, e.g., silica, fluorite, BaF2, and LiF2. Since these glass materials have large dN/dT and expansion coefficients, if a refracting member has a reflective surface, as described above, relatively large quantity of heat generated at the reflective surface will propagate into the refractive member to produce a relatively large temperature distribution inside glass. It is thus considered that there are some measures necessary for suppressing the aberration variation. Under such circumstances that development of thin films is presently under way, particularly, for the exposure light of not more than 200 nm and in conjunction with the fact that the performance expected for the projection optical system itself is also of very high precision in this wavelength range, the aberration variation due to irradiation of the optical members including the reflective surfaces is a significant problem.
Further, in the case of the projection optical system in above-noted U.S. Pat. No. 5,717,518, the beams are considerably concentrated in the finally focused part near the reduced image plane, i.e., in the last optical path in the refracting and reflecting member. Such beams can pose a significant problem, because they produce a heterogeneous large temperature distribution in the optical member.
Even reflecting members sometimes need to be made of a substance with a large expansion coefficient, such as CaF2 or BSC7, because of compatibility with a reflective coat. In such cases a problem will be posed by aberration caused by thermal expansion because of much greater absorptances than those of refracting members.
For avoiding this, it is conceivable to space this refracting and reflecting member apart from the image plane. However, taking it into consideration that the center shield part needs to be designed as small as possible, to space the refracting and reflecting member apart from the image plane will raise considerable difficulties in design. As countermeasures against the aberration due to such heterogeneous temperature distribution, for example, Japanese Patent Application Laid-Open No. H10-242048 notes that rotationally asymmetric aberration variation occurs during execution of scanning exposure or the like and proposes a method and the like for preliminarily fabricating optical means comprising aspherical surfaces in accordance with the aberration variation and moving it as occasion arises. For carrying out this method, however, it is necessary to make the rotationally asymmetric aspherical surfaces according to the aberration variation and to prepare a fabrication system dedicated therefor. In addition, a decentering adjusting mechanism with considerably high accuracy also needs to be incorporated in order to properly decenter the aspherical surfaces during irradiation, which makes the fabrication of the projection optical system difficult.
The aberration variation also occurs with change in the temperature of the entire projection optical system. However, as to the aberration due to deformation of the reflector among such aberration variation, since change of beams due to reflection is approximately four times greater than that due to refraction, influence of temperature change tends to raise a problem in the above-described catadioptric optics. For that reason, it can be said that it is desirable to design the optics, preliminarily taking the deformation of the reflector into consideration.
An example of the countermeasures in this case is the one disclosed in Japanese Patent Application Laid-Open No. H05-144701. This invention is to determine thermal shape changes of some lenses forming the projection optical system and implement optical adjustment with adjusting means, based thereon. It, however, seems considerably difficult in practice to measure the shape changes of the lenses during the exposure operation without negatively affecting the exposure.
In view of the above problems, an object of the present invention is to provide projection exposure apparatus and exposure methods that can readily reduce the aberration variation caused by the temperature change due to the irradiation of the catadioptric optics and other optics with the exposure light.
In order to accomplish the above object, a first projection exposure apparatus of the present invention is a projection exposure apparatus comprising a catadioptric system for focusing an image of a first surface on a second surface, wherein the catadioptric system comprises a member having a reflective surface, said apparatus comprising a heat-transfer member which contains a material with a larger heat conductivity than that of the member having the reflective surface and which is disposed in contact with the member having the reflective surface.
In this case, since the heat-transfer member containing the material with the larger heat conductivity than that of said member having the reflective surface is disposed in contact with said member having the reflective surface, the heat generated at the reflective surface with incidence of the exposure light can be readily radiated to the outside of said member having the reflective surface. Therefore, the heat can be prevented from accumulating inside said member having the reflective surface, and thus it becomes feasible to implement highly accurate exposure with less aberration variation.
In the first projection exposure apparatus of the present invention, it is preferable that said member having the reflective surface comprise an optical member transmitting light and a reflecting plate having said reflective surface, that a surface of said reflecting plate on the side where said reflective surface is formed, have an effective reflection area and be in contact with said optical member, and that said heat-transfer member be in contact with a back face of a region of said reflecting plate in which said effective reflection area exists.
The heat tends to accumulate most in the vicinity of the effective reflection area that actually reflects the light in the reflective surface. When the heat-transfer member is in contact with the back face of the region of the reflecting plate where the effective reflection area exists, as in the present invention, the heat can be radiated from the high-temperature region of the member having the reflective surface to the outside. This permits highly accurate exposure with far less aberration variation.
In order to keep the mentioned optical member from a mechanical load such as pressure occurring in the structure of direct contact of the heat-transfer member with a reflecting optical member having a reflective surface or with a refracting optical member having a reflective surface, it can also be contemplated that the projection exposure apparatus comprises a heat-transfer member with a large heat conductivity placed at a position on the opposite side to a direction of reflection of light on the reflective surface of the optical member and with a predetermined space from the optical member and at least a part of the space between the optical member and the heat-transfer member is filled with a predetermined gas.
In this case, since the space between the optical member and the heat-transfer member is considerably small with the gas in between, the heat at the reflective surface can be radiated to the heat-transfer member with little thermal resistance of the gas. Therefore, it becomes feasible to prevent accumulation of heat inside the optical member and implement highly accurate exposure with less aberration variation. It is also preferable to set the space between the optical member and the heat-transfer member to not more than 30 mm. The reason is that if the space is over 30 mm effective heat transfer to the heat-transfer member will become more difficult, because the heat is not transferred over the heat capacity of the gas.
A preferred form of the above apparatus further comprises forced cooling means connected to said heat-transfer member.
In this case, since said heat-transfer member can be cooled by the forced cooling means, the heat radiation effect is enhanced so as to more effectively prevent increase in the temperature of the member having said reflective surface.
In a preferred form of the above apparatus, said catadioptric system comprises a first reflective surface and a second reflective surface opposed to each other, as said reflective surface, said first reflective surface and second reflective surface have a first transparent portion and a second transparent portion, respectively, capable of transmitting at least part of light in the central part, and at least one of said first reflective surface and second reflective surface is included in said member having the reflective surface.
In this case, the exposure light emerging from the first surface travels through the second transparent portion onto the first reflective surface to be reflected thereby, thereafter is further reflected by the second reflective surface, and then is guided through the first transparent portion to the second surface. On this occasion, since at least one of the first reflective surface and second reflective surface is included in the member having said reflective surface, the heat-transfer member can quickly radiate the heat generated in either of the first reflective surface and the second reflective surface as a result of reflection on the back face by said member having the reflective surface, to the outside of said member having the reflective surface.
A second projection exposure apparatus of the present invention is a projection exposure apparatus comprising a catadioptric system for focusing an image of a first surface on a second surface, wherein said catadioptric system comprises a first reflective surface and a second reflective surface opposed to each other, and a refracting member placed between said first reflective surface and second reflective surface, and wherein at least one surface of said refracting member is provided either with no coat or with three or less coat layers.
In this case, in the refracting member placed between said first reflective surface and second reflective surface, the exposure light passes multiple times between the first reflective surface and the second reflective surface. On this occasion, since at least one surface of the refracting member placed between said first reflective surface and second reflective surface is provided either with no coat or with three or less coat layers, absorption at the transmissive surface of the refracting member can be controlled to the minimum. This makes it feasible to prevent accumulation of heat in the refracting member and to implement highly accurate exposure with less aberration variation.
In a preferred form of the above apparatus, at least one of said first reflective surface and second reflective surface has a positive power, said first reflective surface and second reflective surface have a first transparent portion and a second transparent portion, respectively, capable of transmitting at least part of light in the central part, and a shield plate for absorbing light entering said catadioptric system is disposed in a part of the space between said first transparent portion and second transparent portion.
In this case, the exposure light emerging from the first surface travels through the second transparent portion onto the first reflective surface to be reflected thereby, thereafter is further reflected by the second reflective surface, and then is guided through the first transparent portion to the second surface. On this occasion, since the shield plate for absorbing the light entering said catadioptric system is disposed in a part of the space between said first transparent portion and second transparent portion, it becomes feasible to suppress appearance of flare caused by reflected light on the transmissive surface of the refracting member placed between the first reflective surface and the second reflective surface, with effectively utilizing the exposure light.
A third projection exposure apparatus of the present invention is a projection exposure apparatus comprising a catadioptric system for focusing an image of a first surface on a second surface, wherein the catadioptric system comprises a first reflective surface and a second reflective surface, at least one of the first reflective surface and second reflective surface has a positive power, said first reflective surface and second reflective surface have a first transparent portion and a second transparent portion, respectively, capable of transmitting at least part of light in the central part, at least one of said first reflective surface and second reflective surface is formed in an end face of a transmissive member, and the following condition is met:
S2/xcfx8622 less than 3S1/(xcfx8612xe2x88x92xcfx8622)xe2x80x83xe2x80x83(1), 
where xcfx861 is an effective diameter and S1 an absorptance on the occasion of incidence of light to the reflective surface formed in the end face of said transmissive member, and xcfx862 an effective diameter and S2 an absorptance on the occasion of incidence of light to a transmissive surface kept in contact with the reflective surface formed in the end face of said transmissive member.
In this case, the exposure light emerging from the first surface travels through the second transparent portion onto the first reflective surface to be reflected thereby, thereafter is further reflected by the second reflective surface, and then is guided through the first transparent portion to the second surface. On this occasion, since above Condition (1) is met at the first reflective surface or at the second reflective surface, absorption of the exposure light, i.e., quantities of heat generated are balanced to some extent between the reflective surface and the transmissive surface. This can prevent the transmissive member from being locally heated to cause heterogeneous aberration variation, whereby it becomes feasible to implement highly accurate exposure.
A fourth projection exposure apparatus of the present invention is a projection exposure apparatus comprising a projection optical system for focusing an image of a first surface on a second surface, wherein said projection optical system comprises at least one reflector and an expansion coefficient xcex2 of at least one of materials making said reflector satisfies the following condition:
xcex1/3 less than xcex2 less than 3xcex1, xcex1xe2x89xa0xcex2xe2x80x83xe2x80x83(2) 
where xcex1 is a coefficient of thermal expansion, (dL/L)/dT, of a barrel of said projection optical system.
In this case, since the expansion coefficient xcex2 of at least one of the materials making the reflector satisfies above Condition (2), the expansion coefficients of the reflector and the barrel can be made approximately equal to each other. Namely, since the size of the barrel varies according to change in the focal length due to the heat generated in the reflector, variation can be suppressed in the focusing condition of the projection optical system and it thus becomes feasible to implement highly accurate exposure.
Exposure methods of the present invention are exposure methods using the above projection exposure apparatus according to either of the above aspects of the present invention, which comprise a step of generating illumination light, a step of placing a mask with a predetermined pattern formed therein, on the first surface and illuminating the mask with said illumination light, and a step of projecting an image of said predetermined pattern of said mask placed on said first surface, onto a photosensitive substrate placed on said second surface.
In this case, use of the projection exposure apparatus according to either of the aspects of the present invention permits prevention of the aberration variation of the projection optical system and the variation of the focus condition, whereby it becomes feasible to implement highly accurate exposure.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.